The present invention relates generally to paper making and, more particularly, to synthetic press rolls for use in paper machines and to methods for manufacturing such synthetic press rolls.
Rock rolls used in paper machine press sections are formed of granite for the reason that the surface properties of granite provide for a controlled detachment of the paper web from the surface of the rock roll. Granite also provides good resistance to the abrasive effects of a doctor.
However, granite does have some drawbacks. Being a natural material, its properties tend to vary which may result in non-uniform, and in certain cases even inadequate, detachment of the web from the surface of the roll. Internal flaws may exist in the granite material which result in a tendency for the roll to crack. Indeed, this tendency to crack constitutes the most serious problem in applications where the rock roll is subjected to high linear loads at high temperatures, such as where it is attempted to increase the dry solid content of the paper in the press section of the paper machine. It is advantageous to increase the dry solid content of a web in the press section of a paper machine since this is significantly more economical than dewatering a web by evaporation. Moreover, a web which has been dewatered to a greater extent in the paper machine permits the running speed of the paper machine to be increased, and the most natural way of increasing the dry solid content of the web is to increase the linear load and the temperature at the press section. However, this has not been possible to the extent desired using conventional rock rolls. Another disadvantage of conventional rock rolls is that they are heavy components which increase the tendency for the paper machine to oscillate during operation. The great weight of rock rolls also requires more extensive frame constructions for the paper machine.
Conventional rock rolls generally comprise a relatively thick cylindrical mantle of granite inside of which a steel shaft is situated. Tensioning flanges are attached at the ends of the steel shaft by means of which the granite mantle is compressed to eliminate the possibility of tensile loads acting on the mantle under any loading condition. Such construction is relatively complicated and it is desirable in any modification of a press roll to retain such basic construction.
Attempts have been made to provide synthetic rock rolls. Generally, such rolls are provided with a polymer surface to which a rock powder, such as quartz sand, has been added to hard rubber or polyurethane. These prior art synthetic rock rolls have the drawback that the paper web has a tendency to adhere excessively to the surface of the roll and, moreover, the roll has poor strength properties. In particular, the use of such polymer-surfaced rolls at elevated temperatures, such as above 80.degree. C., has not been possible.